1. Field of the Invention
This invention relates to an electrophotographic transfer apparatus and more particularly to an improved electrophotographic toner transfer apparatus of the type which employs a transfer web or belt.
2. Description of the Prior Art
In customary electrophotographic processes, a conductive backing having a photoconductive insulating layer thereon is electrostatically imaged by first uniformly charging its surface, and subsequently exposing the charged surface to a pattern of activating electromagnetic radiation, such as light. The radiation pattern selectively dissipates electrostatic charges in the illuminated areas on the photoconductive surface, which results in a latent electrostatic image in non-illuminated areas. This latent electrostatic image can be subsequently developed to form a visible image by depositing solid electrophotographic developer materials thereon by a variety of development techniques, the most common of which is cascade development. The developed toner image is typically transferred and fused to another substrate such as plain paper.
Transfer of the toner image to the paper can be achieved in a number of ways, and one such method involves the use of an intermediate transfer web or belt. See, for example, Carlson, U.S. Pat. No. 2,990,278; Byrne, U.S. Pat. No. 3,591,276; and, co-pending U.S. patent application Ser. No. 403,696, filed Oct. 4, 1973. Although the term "web" will be used hereinafter, it is used to include belts and other similar transfer members.
A serious problem which has developed in use of intermediate transfer webs is the requirement to accurately track the web. Such webs are usually driven by a series of rollers, and their longitudinal alignment is often thrown off because the web tends to "walk" around the rollers. This effect is cumulative and it interferes with accurate registration of the system. Additionally, webs tend to wander from side to side on their rollers which also results in misalignment problems.
Methods are known for reducing alignment errors, or for correcting them. Heretofore, however, such methods have either been complicated or impractical. For example, one solution involves the utilization of crowned rollers to drive the belt which reduces tracking errors. Nevertheless, such crowned rollers do not totally eliminate misalignment problems, and since the effect is cumulative, the amount of registration error introduced becomes significant after many cycles, even though the error per cycle is exceedingly small. Additionally, crowned rollers make belt replacement and maintenance a complicated operation.
Alternatively, electronic sensing means can be used to detect misalignment errors. As would be expected, such electronic means are relatively expensive and usually require the copier to be shut down after a tracking error of a certain magnitude is detected so that an operator can manually correct the error. This requirement for frequent servicing with concomittant down-time is highly undesirable in the office copier field.